1. Field of the Invention
The present invention relates to an exposure method in an optical lithography process used in a semiconductor device manufacturing process, a phase-shifting mask used in an exposure system, and a method of manufacturing the amount of spherical aberration of a projection lens system in an exposure system.
2. Description of the Related Art
At present, an optical lithography technique has been used to form a pattern on a semiconductor substrate in a semiconductor device manufacturing process. In the optical lithography technique, a pattern of a photomask (which is a master plate for exposure on which a pattern containing a transparent area and a light shielding area is formed. It is called as "reticle" particularly when the reduction ratio is not 1:1, however, in the following description it is referred to as "photomask" or merely "mask" in any case) is transferred onto a semiconductor substrate coated with photosensitive resin by a reducing projection exposure system, and then developed to form a predetermined pattern of photosensitive resin.
The optical lithography techniques until now have been mainly advanced to develop exposure systems, and particularly to increase NA (numerical aperture) of the projection lens system and thus enhance the minuteness of the semiconductor device pattern. Here, NA represents the numerical value which represents the degree of spreading of light which can be converged by the lens. As the numerical value is larger, more spreading light can be converged and the lens performance is higher.
Further, as generally well known as Rayleigh equation, the resolution limit R (the limit dimension of a fine pattern which can be resolved) and NA has the following relationship: R=K.sub.1 .times..lambda./NA (here, K.sub.1 is a constant dependent on the process such as performance of photosensitive resin or the like), and the resolution limit is more minute as NA is increased.
However, although the resolution is enhanced by increasing NA of the exposure system, the depth of focus (the range in which the deviation from the focus position is permissible) is reduced, and thus it is difficult to achieve a more minute design from the point of the depth of focus. As a physical description is omitted, it is also well known that the depth of focus DOF and NA have the following relationship: DOF=K.sub.2 .times..lambda./NA.sup.2 (here K.sub.2 represents a constant dependent on the process) as Rayleigh equation as like the foregoing case. That is, as NA is increased, the depth of focus is narrower, and thus no permission is given even to a slight deviation from the focus position.
Therefore, considerations on the spherical aberration have been made to enlarge the depth of focus. The effect of the considerations of the spherical aberration is described in detail in Japanese Laid-open Patent Application No. Hei-2-166719. When the spherical aberration is subjected to excessive correction, the contrast in the best focus state is reduced, however, the deterioration due to defocus is suppressed, so that the depth of focus can be enlarged. Further, it is known that the spherical aberration can be varied by varying the optical path between a mask and a projection lens system. As one of specific methods, Japanese Laid-open Patent Application No. Hei-2-166719 discloses a method of designing a projection lens to have a telecentric structure even at a mask side and inserting a transparent plane-parallel plate between the mask and the projection lens system. If the plane-parallel plate is inserted into a portion where light is made telecentric, only the spherical aberration can be varied without effecting the other aberrations. Further, as a conventionally well known method has been used a method of varying the spherical aberration by moving the mask to be close to or far away from the projection lens. Still further, Japanese Laid-open Patent Application No. Hei-6-97040 discloses that the sign of the spherical aberration to be applied is selected in accordance with the positive or negative type of photosensitive resin.
The spherical aberration is not altered while it is used on a semiconductor device manufacturing line except for an exposure system having a spherical aberration correcting mechanism, and the amount of the alteration is not measured. Aberration such as distortion, curvature of field or coma is periodically measured, and managed so that the value thereof is below a predetermined value. However, the spherical aberration is little effective, and thus it is considered unnecessary to manage spherical aberration. In addition, there is no method of measuring the spherical aberration of the projection lens which is installed in an exposure system. Therefore, no variation has been made on the amount of the spherical aberration of the installed projection lens. In general, the spherical aberration of the projection lens is adjusted before the projection lens is installed into the exposure system and, thereafter the projection lens is used without any adjustment as is installed into the exposure system. On the other hand, in the exposure system having the spherical aberration correcting mechanism, the spherical aberration amount to be added due to the variation amount of the optical path between the mask and the projection lens when an aberration varying mechanism is moved is calculated.
Separately from the above-mentioned enlargement of the depth of focus by the spherical aberration, various super-resolution methods have been considered. In general, the super-resolution method is a method of improving the light intensity distribution on an image plane by controlling the transmittance and the phase in an illumination optical system, a photomask and the pupil plane of the projection lens system.
A phase-shifting mask which is used in performing a super-resolution method based on the improvement at the photomask side will be described hereunder.
The phase-shifting mask is used to control the phase of light passing through the mask to improve the light intensity distribution on the image plane.
There are various types for the phase-shifting mask. First, the phase-shifting mask of Shibuya-Levenson type disclosed in Japanese Laid-open Patent Application No. Sho-57-62052 will be described. This type is a system of alternately changing the phase of light passing through a transparent area in a periodical pattern by 180 degrees.
FIGS. 1A and 1B are plan view and longitudinally-sectional view which show a Shibuya-Levenson type phase-shifting mask. A light-shielding film 102 is formed on a transparent substrate 101, and the light-shielding film 102 is selectively removed to form a space pattern 11 (openings) periodically. A transparent film 104 is disposed in every other space pattern. The wavelength .lambda. of light is represented by .lambda./n in a medium in which the light is propagated (n represents refractive index of the medium), and thus a phase difference occurs between light passing through air (n=1) and light passing through the transparent film 104. The phase difference is set to 180 degrees by setting the film thickness t of the transparent film 104 to t=.lambda./2(n.sub.1 -1) (here, .lambda. represents the wavelength of exposing light and n.sub.1 represents the refractive index of the transparent film 104).
Therefore, as shown in FIG. 1C, the amplitude distribution of the transmission light through the Shibuya-Levenson type mask has such a distribution that the phase is alternately inverted, and this amplitude distribution has a period which is twice of that of the original distribution. Therefore, the diffraction angle of the diffracted light from the mask is set to a half of the usual one. The light diffracted by the mask having a pattern which is so fine as below the resolution limit of the prior art can also be collected through the projection lens. Due to the interference between light beams which are inverted in phase, the light intensity is reduced between adjacent opening portions, whereby a fine pattern can be separated. The transparent film 104 is called as a phase shifter, and it is usually formed of silicon oxide (SiO.sub.2).
As another type of the phase-shifting mask has been known an auxiliary pattern type which is applicable to an isolated pattern as disclosed in Japanese Laid-open Patent Application No. Sho-62-67514. In this mask, a fine pattern which is not resolved is provided around an original pattern (hereinafter referred to as "main pattern"). The phase of light is inverted between the main pattern portion and the auxiliary pattern portion to give the effect of the phase shift.
FIGS. 2A is a plan view showing an auxiliary pattern type phase-shifting mask, and FIG. 2B is a longitudinally-sectional view showing the auxiliary pattern type phase-shifting mask shown in FIG. 2A. As shown in FIGS. 2A and 2B, a space pattern 11 which is an original pattern to be transferred and an auxiliary pattern 12 which is a fine pattern below the resolution limit of the exposure device are formed on a light-shielding film 102 on a transparent substrate 101. Further, a transparent film 104 is formed on the auxiliary pattern 12, whereby a phase difference of 180 degrees is provided between light passing through the space pattern 11 and the light passing through the auxiliary pattern 12.
Still further, a halftone type of phase-shifting mask disclosed in Japanese Laid-open Patent Application No. Hei-2-256985, to which much attention is paid at present, is known as being simple in the mask design and manufacturing process. The considerations on the halftone type have been mainly advanced for hole patterns. It is also shown that the combination with the halftone type and a modified illumination method also gives an effect on a general line type pattern.
FIGS. 3A and 3B show a halftone type phase-shifting mask. As shown in FIGS. 3A and 3B, a semi-transparent film 103 is provided in place of the light-shielding film of the usual mask so that a phase difference of 180 degrees occurs between light passing through the semi-transparent film 103 and light passing through the transparent area around the semi-transparent film 103. The semi-transparent film is formed of chromium oxide nitride, molybdenum silicide oxide nitride, chromium fluoride or the like, and the transmittance thereof is generally set to the range from 4% to 20%.
The Shibuya-Levenson type and the halftone type were first applied to the X-ray exposure field. In the X-ray exposure is used a mask which has a pattern formed with heavy metal such as gold, tungsten or the like on a thin film such as SiC, diamond or the like which is called as "membrane". SiC or the like has little absorption of X-rays, and most of X-rays is shielded by the portion on which the heavy metal pattern is formed, and thus the pattern can be transferred. However, heavy metal must be formed at a relatively large thickness in order to perfectly shield the X-rays, and the increase in film thickness makes the processing thereof difficult. Therefore, light is somewhat transmitted through the mask. However, it has been also found that the contrast is more enhanced than the case where the light is perfectly shielded if the phase difference is equal to 180 degrees even when light is somewhat transmitted. This was the first proposal of the halftone phase-shifting mask.
With respect to the X-ray exposure, there have been considered a proximate exposure system for performing a transfer process at equi-magnification while a semiconductor substrate and a mask are brought into close contact with each other at a distance of several microns, and a reductive exposure system for performing a transfer process under demagnification by using a mirror which is formed by multi-coating materials having different refractive indexes. Further, a transmission type and a reflection type are known as the type of the mask. The above-mentioned principles of the phase shift are applicable to both types in the same manner.
The optimum phase difference of the phase-shifting masks as described above is 180 degrees on principle, and the exposure characteristic is deteriorated if there is a phase error (a deviation from 180 degrees in phase difference). It is known that the phase error has the most remarkable effect on the focus characteristic (the relationship between the focus position and the pattern dimension) and the focus characteristic is inclined by the phase error. In general, the permissible range of the phase error is set within .+-.5 degrees.
Further, a phase difference measuring machine which is exclusively used for phase-shifting masks has been developed in order to accurately manage the phase difference. At present, Phase-1 produced by Mizojiri Optics Company (Japan) and MPM-100/248 produced by Lasertec Corporation (Japan) are introduced into many phase-shifting mask manufacturing lines, and they are used as standard machines. In these measuring machines, light having the same wavelength as the exposure light is used, and one light beam is divided into two light beams to transmit the two light beams through different places on the mask and then make these transmitted light beams interfere with each other, thereby measuring the phase difference between the transmitted light beams at the two places.
However, as indicated by the inventors of this application in Japanese Laid-open Patent Application No. Hei-8-114909, there is a case where it is better to intentionally generate a phase error. Here, when the phase difference is equal to 180 degrees, it may not meet the optimum condition due to the effects of the structure of the semiconductor substrate, the thickness of the photosensitive resin film, the solubility characteristic of the photosensitive resin, the aberration of the projection lens system, etc.
The conventional phase-shifting mask and exposure method as described above have the following problems.
(1) If the setting of the phase difference is displaced in the phase-shifting mask, the focus characteristic would be inclined. Since the phase difference is dependent on the thickness and the refractive index of the transparent film or the semi-transparent film, a phase error occurs due to the following two causes: 1) the variation of the refractive index, and 2) the dispersion of the film thickness. The phase difference cannot be accurately measured unless the transparent film or the semi-transparent film is processed and the phase-shifting mask is finally completed. In general, the film formation by a sputtering method causes variations of the film thickness by several percentages, and thus a phase error of about 5 degrees in phase difference remains. This phase error can be suppressed to a small value by manufacturing a number of masks and selecting from these masks a mask whose phase is nearest to a desired one. However, in this method, the price of the phase-shifting mask is heightened, and thus it is not practically usable.
(2) There is a case where the optimum phase difference is not equal to 180 degrees in a phase-shifting mask, and thus in the prior art it is needed to determine the optimum phase difference on the basis of the result of experiments which are made by actually using the semiconductor substrate, the phase-shifting mask and the exposure system.
(3) In the exposure method, the effect of the spherical aberration is little, and the accurate measurement of the spherical aberration is not performed. Therefore, there is no simple measuring method which can measure the spherical aberration amount of the projection lens system installed in the exposure system.